Formlet

Formlet is a term used to describe a small, self-contained form used within a user interface to collect a specific set of input data. In web development, a formlet typically includes fields, client-side validation, and submission logic, and is designed to be embedded inside larger pages or wizards, often as a reusable building block in a modular UI.

In the field of programming languages, Formlets refer to a formal abstraction introduced in the early 2000s by researchers including Erik Meijer, to model forms as composable, typed values. A form is built from smaller components using combinators, and the result is a value that represents the collected data, with validation expressed as part of the type or applicative structure. The approach aims to provide static guarantees about input requirements and to simplify construction of complex forms by composition rather than imperative code.

Over time, the concept influenced modern composable UI patterns and form-building libraries, although contemporary libraries use different terminology. The core ideas—modularity, declarative composition, and separation of data capture from processing—remain common in form design.